Radiation Basics

What is a Contrast Agent

Contrast agents, also known as "dyes," are used
to highlight specific organs, blood vessels, or tissues and make
them easier for the radiologist to determine the extent of disease
or injury.
The most common contrast agents that are used include:

Iodine

Barium sulfate

Gastrografin

Gadolinium

These may be administered in three different ways:

By injection into the vein (intravenous)

By mouth

Through the rectum

Intravenous Contrast

Intravenous contrast is used to highlight blood vessels and to
enhance the structure of organs like the brain, spine, liver, and
kidney. The contrast agent (usually an iodine compound or
gadolinium) is clear and is injected through a small needle taped
in place (usually on the back of the hand) during a specific period
in the imaging exam.

Once the contrast is injected into the bloodstream, it
circulates throughout the body. When the test is finished, the
kidneys and liver quickly eliminate the contrast from the body.

Iodine is considered to be a safe contrast agent but some
individuals can have an allergic reaction that can be severe or
fatal. Because iodine contrast increases the visibility of target
tissues on the images, the benefits are considered to outweigh the
risks but discuss this option first with your doctor to determine
if this is the best choice for you.

Gadolinium is often used as a contrast agent for MRI tests.
While it is considered largely free of side effects, some may
experience side effects that are mild to severe. As with iodine
mentioned above, it is always best to discuss the possible risks
with your physician.

Oral Contrast

Oral contrast is used to highlight gastrointestinal (GI) organs
in the abdomen and pelvis. If oral contrast will be used during an
examination, the patient will be asked to fast for several hours
before administration.

Two types of oral contrast are used:

Barium sulfate, the most common oral contrast agent, resembles a
milk shake in appearance and consistency. The compound, available
in various flavors, is prepared by mixing with water.

Gastrografin is a yellowish, water-based drink mixed with
iodine. It can have a bitter taste.

When oral contrast has been requested by the doctor, patients
usually drink the equivalent of three or four 12-ounce
drinks.
After the contrast is swallowed, it travels to the stomach and
gastrointestinal tract. Like intravenous iodine, barium and
gastrografin highlight the organs that have responded to the
contrast as white areas.

In general, both barium and gastrografin contrast are safe and
pass through the gastrointestinal tract but some patients may
experience mild to severe side effects. Discuss the benefits versus
risks with your physician.

Rectal Contrast

Rectal contrast is used when enhanced images of the large
intestine and other lower GI organs are required. The same types of
contrast used for oral contrast are used for rectal contrast, but
in different concentrations.

Rectal CT contrast is usually administered by enema. When the
contrast is administered, the patient may experience mild
discomfort, coolness, and a sense of fullness. After the CT is
complete, the contrast is drained and the patient may go to the
bathroom.

The preparation for rectal contrast is similar to oral contrast,
in that the patient should be fasting for several hours before the
test. In addition, the patient will be required to use an enema to
cleanse the colon the night before the examination.